fix: Notify build_runner of dependency on inputSpec

[PvtPuddles]

Description

When the builder loads the inputSpec, I have added a quick call to buildStep.canRead. This notifies build_runner that the generator depends on the inputSpec file, causing the runner to be called whenever the spec changes.

Notes:

• I did not remove the existing work-around; this fix only works when the API Spec lives inside the lib directory (and this would be a breaking change).
• I have not updated documentation; I will wait for the green light on the code change first.

Type of Change

• ✨ feat -- New feature (non-breaking change which adds functionality)
• 🛠️ fix -- Bug fix (non-breaking change which fixes an issue)
• ❌ ! -- Breaking change (fix or feature that would cause existing functionality to change)
• 🧹 refactor -- Code refactor
• ✅ ci -- Build configuration change
• 📝 docs -- Documentation
• 🗑️ chore -- Chore